The present invention relates generally to liquid crystal display devices, such as a microdisplays and more particularly to an apparatus and methods for generating an accurate, visually appealing black frame around the image being displayed on either a display screen or for a near to the eye or binocular application microdisplays.
Microdisplays are a type of liquid crystal display (LCD). LCDs are commonly used in portable televisions, portable computers, and cellular phones to display information to the user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state and block the transmission of light in a second state. When used as a high resolution information display, as in the application of the present invention, the LCDs are typically arranged in a dot-matrix configuration with independently addressable pixels. Each individual pixel is controlled to selectively modulate light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode). The matrix of pixels is laid out on a semiconductor substrate or die, which is produced from a semiconductor wafer or other suitable substrate such as a silicon wafer. Multiple die can be generated from a single wafer or substrate.
Typically, the image generating portion of the microdisplay, is constructed by bonding a piece of glass or transparent material G to an electrical circuit fabricated on a semiconductor or other suitable substrate chip S, as illustrated in FIG. 1. The glass or transparent material G is held to the silicon S by a perimeter seal PS, which is generally formed of an epoxy or other similar sealant-type material. Liquid crystal LC fills the region between the glass or transparent material G and semiconductor substrate or silicon S, and inside the perimeter seal PS, as also shown in FIG. 1. The region inside of the perimeter seal is generally considered to be available as the viewing or active area of the display, i.e., this is the area where the pixels and liquid crystal are located.
A drawback of this construction is that when an image is displayed upon a screen or viewed directly, from the microdisplay; the perimeter seal PS, and any space between the inside of the perimeter seal PS and outer perimeter of pixels, are visible. In a projected or viewed image, this region of the display is not visually appealing. Another drawback of this construction is that stray light bounces off of the perimeter seal PS surfaces and surfaces adjacent to the perimeter seal PS. This can lead to a reduction in the contrast ratio of the display, which is undesirable.
In one solution to the above described problems in the prior art, a black frame BF is placed on the top surface of the glass G. In one embodiment of this solution, the frame is formed by cutting a piece of metal coated black into the desired pattern. The metal frame is then adhered to the top surface of the glass G. Alternatively, the frame is formed by depositing a black ink on the top surface of the glass G in the desired pattern. The black color of the coating or ink absorbs stray light SL, and thus prevents the image of the perimeter seal PS from being visible, as shown in FIG. 2. Any other light L that is directed onto a pixel P will be reflected back. Thus, as long as the opening in the frame is made so that the perimeter seal PS (and any other aspects of the image that are not desired to be projected) is protected from incident light, the protected image will be masked.
There are, however, a number of drawbacks to the Black Frame and Black Ink solutions. One is the parallax of incoming light. Another is Snell""s Law. Parallax refers to the phenomenon where lightwaves which enter the glass are not perpendicular to the surfaces of the glass. These light rays strike the interior and exterior faces of the glass and the pixels on the Fabricated Semiconductor at an angle and are reflected back out of the glass at a complementary angle.
With the phenomenon described by Snell""s Law, light waves are bent as they travel through glass to air and glass to LC fluid boundaries, in each direction, into and out of the glass.
Because light rays must travel through the thickness of the glass G before striking the pixel array, the impacts of parallax and Snell""s Law must be accounted for. FIG. 3 illustrates these impacts as it travels through the display.
The primary aspect of the parallax problem results from the fact that there is some finite distance, D between the edge of the black frame BF and the first illuminated pixel P due to the distance the light must travel through the glass G. See FIG. 3. By calculating this distance, it is possible to develop an opening size in the black frame BF that will allow the appropriate region of the display to be illuminated, so that the perimeter seal PS is covered, but the pixels are left open. However, if the angle of incidence of the outermost light rays varies, which it often does because of the incident angle and f# of each different lens system, the region of the display that is illuminated changes.
Two different prior art solutions were developed to deal with this latter problem. One involves creating a custom black frame opening and location for each different optical lens system. A drawback of this solution is that it requires customization of what is an otherwise standard product, which increases product cost and complexity. Another solution to this latter problem, involves increasing the number of pixels on the silicon. In this arrangement, xe2x80x9cunusedxe2x80x9d pixels adjacent to the perimeter seal or black frame are driven black. A drawback of this solution is that it reduces the number of displays that can be produced from each wafer because each silicon die is increased in size. Another drawback of this latter solution is that xe2x80x9cblackxe2x80x9d pixels BP are not truly black. This is because of the contrast ratio of the display. Thus, with this latter solution, a gray region GR is projected between the black frame BF and the projected pixels PP. This effect is illustrated in FIG. 4.
The major disadvantage of the solution employing a black frame or printing black ink on the top surface of the glass is that the tolerances in the size of the opening of the black frame. The positioning of the black frame poses additional problems of requiring 20 or more additional pixels on each side of the display to compensate for these inaccuracies. These additional pixels not only decrease the yield from each wafer, but they also create the gray ring to be displayed in the projected or observed images described above, depending upon the microdisplay application.
Yet another disadvantage of this solution is that because the black frame is outside of the focal plane of the optics system (the focal plane is at the surface of the silicon die), the edges are slightly blurry.
Another problem with using black frames is thermal management. The black frame absorbs energy, and thus becomes hot. Removing the energy, particularly in high energy lighting projection systems, using microdisplays, without causing it to damage the liquid crystal, is problematic.
Still another drawback of using black frames, applied on the top surface of the glass, is that manufacturing such devices is expensive. Each frame has to be added to the microdisplay one at a time. Because alignment tolerances are critical, this step thus requires a lot of time and specialized equipment.
Another aspect of microdisplay devices relevant to the present invention relates to the connection of what is known as the VCOM or ITO signal. This is a voltage that exists on a thin layer of transparent metal, ITO (Indium Tin Oxide), which is coated on the inside surface of the glass G. This voltage sets up the potential difference across the liquid crystal. It provides the reference voltage for the pixel voltage on the silicon S. It is important that the connections to the ITO be of low resistance so as to provide low power consumption and high contrast displays.
Typically, connectivity to the ITO layer I on the inside surface of the glass is accomplished by employing conductive solders or epoxies E, which connect to the circuit (usually by a flex circuit or printed circuit board) C to the ITO layer I on the exposed ledge of the glass, as shown in FIG. 5. The resistance of the electrical path going through the conductive epoxy E and directly into the ITO layer I is relatively high. This is because conductive epoxies have marginal electrical bond capabilities with the ITO layer I. The resistance of ITO coatings varies with the type and thickness, but a general trend is that optically favorable ITO coatings tend to have higher resistance values. As microdisplays begin using these optically favorable coatings, the contact resistance will increase further. This increased contact resistance requires higher driving voltages for the VCOM signal, in turn leading to more expensive electronics and/or shortened battery lives for hand-held devices. It is important that any solution, which is proposed for improving the generation of the black frame of the visual display panel, not increase the contact resistance between the ITO layer I and the circuit interconnect C.
The present invention is directed to overcoming, or at least minimizing the drawbacks of the prior art liquid crystal display devices.
In one embodiment of the present invention, a LCD having an improved black frame is provided. The microdisplay of the LCD according to the present invention has a matrix of pixels disposed on the top surface of a silicon substrate. It also includes a layer of transparent material, preferably glass, having a top surface and a bottom surface. The bottom surface of the layer of transparent material is disposed adjacent to the top surface of the silicon substrate. A seal is disposed between the silicon substrate and the layer of transparent material. The seal is arranged to form a perimeter around the matrix of pixels. An annulus, is formed by a region bounded by the top surface of the silicon substrate, the bottom surface of the layer of transparent material, and the area inside of the perimeter formed by the seal. The annulus is filled with a liquid crystal. The liquid crystal which when activated (exposed to a voltage potential), twists, changing the polarity of any light directed on it.
This embodiment of the present invention further includes a layer of reflective material positioned between the top surface of the semiconductor substrate or back plane and the bottom surface of the layer of transparent material. The layer of reflective material is patterned to form a frame around the matrix of pixels. The layer of reflective material preferably includes a metal or metal alloys, e.g., aluminum, platinum, chromium, copper, silver or any other metal or metal alloys having a reflectivity of at least 97%. This embodiment of the present invention also includes a layer of conductive material that is disposed on the bottom surface of the layer of transparent material. The layer of reflective material is preferably disposed on the layer of conductive material. This embodiment further includes a plurality of transparent beads disposed in the annulus. The plurality of beads act as spacers to prevent the top surface of the semiconductor substrate from coming in contact with the layer of conductive material on the bottom of the glass or transparent material.
In another embodiment of the present invention, a light absorptive layer is placed over the top surface of the layer of transparent material. The light absorptive layer in this embodiment is also patterned to form a frame around the matrix of pixels. Preferably, this light absorptive layer is comprised of a structure or material with black chromium, ink or other suitable coating. In yet another embodiment of the present invention, a light absorptive layer is placed on the top surface and the reflective layer is placed on the bottom surface of the transparent material.
The present invention differs from the prior art solutions in a number of respects. First, in the preferred embodiment, it places the frame at the surface of the liquid crystal, which is at the focal plane of the optical system. This eliminates parallax, which means that a single display can be created that will work in many different optical systems. It also reduces the number of required pixels, and allows the die size to be minimized, which decreases the cost of the display. Furthermore, because the reflective frame utilized in the present invention reflects light rather than absorbs it, it does not generate heat in the device, thus eliminating thermal management as an issue.
Furthermore, with the present invention, since the frame is placed on the inside surface of the glass, it enables a photolithography process to be used to create the frame. This allows for much tighter tolerances than can be attained with a metal or pad printed frame, which in turn can lead to a smaller die size. Also, during manufacturing, the masks used in the present invention to form the reflective pattern used in generating the black frame can be placed on multiple displays at a time. A single sheet of glass can be used to make 50 to 100 displays screens. This has a significant improvement on throughput and thus decreases manufacturing costs.